1. Field of the Invention
The invention relates to a new process for the preparation of dialkyl carbonates by reacting carbon monoxide with alkyl nitrites in a continuous gas phase reaction over a catalyst.
Dialkyl carbonates are of general chemical importance. Thus, for example, diethyl carbonate is an excellent solvent in the middle boiling range. Furthermore, the dialkyl carbonates are excellent carbonylation and alkylation reagents. Finally, they have acquired great importance in the preparation of other carbonates and of urethanes and ureas.
2. Description of the Related Art
Various methods for the preparation of dialkyl carbonates are described in the literature. In this regard reactions of phosgene or alkyl chloroformates with alcohols (Ullmann's Encyclopaedia of Industrial Chemistry, Vol. A 5 (1986), 197-202) have been of considerable industrial importance up to the present time. There is, however, an increasing interest in replacing, by other processes, the use of phosgene, which is toxic, or of the intermediate products derived therefrom, such as chloroformic acid esters. Thus the preparation of dialkyl carbonates by reacting carbon monoxide with lower alcohols in the presence of various catalysts and co-catalysts is known. In accordance with EP 365,083 lower alcohols are reacted with carbon monoxide in the presence of oxygen over a catalyst system composed of a copper alkoxyhalide and water. In accordance with EP 354,970 lower alcohols are also reacted with carbon monoxide in the presence of oxygen over a catalyst system consisting of a combination of a copper catalyst or a platinum metal catalyst and an alkaline earth compound, both the copper and the platinum metal and also the alkaline earth metal being employed in the form of a salt of a weak acid or in the form of a halide.
The reaction of an alkyl nitrite with carbon monoxide in the gas phase over a supported platinum metal catalyst is disclosed in JP 60-181, 051 (1985) (C.A. 104 (1986, 110 355 t)), the reaction being carried out in the presence of 10 mol %, relative to CO, of an oxidizing agent, for example oxygen. Furthermore, it is disclosed in EP 57,629 that an alkyl nitrite, which is prepared in situ from the appropriate alcohol and an oxide of nitrogen, is reacted with CO in a gas phase process in the presence of a supported palladium catalyst to give a dialkyl oxalate.